I agree that consistency in everything is absolutely key. Until I got the hang of daily tuning to a consistent interval, progress was almost impossible. As you'll know, the weather in the UK can change from heatwave to frostbite in 24 hours, and, as this effects the tuning, I found that aerial-fingering/muscle memory that had worked well one day could be inaccurate the next day. Whenever that happened it felt like I was starting over and having to relearn, and, even if I managed to retune the theremin, it often hindered my practice and dented what little confidence I'd gained. It was only once I got used to daily tuning to a precise interval that I felt able to make any (small) advancement at all.
From what I've observed, it seems that, despite certain "schools" of aerial-fingering, no two thereminists appear to have an identical style of play. I guess everyone champions the method they prefer leading to inevitable ongoing debate. Personally, I wonder if everyone has to find what "fits" right for them as an individual, so there's probably no harm in experimenting in the early stages; I know I did. Personally, I found knuckle extending didn't work for me, although it's probably one of the most efficient ways of playing. My problem was that I broke my little finger several years ago, and it has an annoying habit of "popping" from fully bent to fully open; this is beyond my control, and cannot be rectified by practice, (despite the fact that as a lifelong pianist I used to be able to move it entirely independantly of my other fingers). This "jumping" litle finger made the knuckle-extending method unworkable for me. I also tried an open-handed method, which, although I found it easier for larger intervals, was much less stable generally. Over a period of time, I settled into a style of aeriel-fingering that felt natural and seemed to work ... although I still have much to learn.
Finally, you mentioned about playing at a slight upward angle towards the antenna. I've observed that people play both at an angle or straight, but I guess whatever you chose, (or feels right) you must be consistent about it. With non-linear theremins, (where the notes intervals are narrower closer to the antenna) I personally find that there's a line or path of optimal play which, for me, seems to curve upwards towards the antenna, in a way that allows me to use more or less consistent fingering over narrower intervals by adjusting the angle of play, (if that makes any sense; I've possibly not explained myself very clearly).
Above all things, I'd say that the most important is to enjoy yourself, have fun, and don't let anyone discourage you!